This invention relates to a digital tuning receiver of the type including a phase locked loop (hereinafter referred to as "PLL") synthesizer. In particular, it relates to a receiver wherein a tuning operation can be correctly carried out and a muting operation can be correctly carried out in accordance with the tuning operation.
Radio receivers of the type including PLL synthesizers have been extensively used in FM stereophonic receivers. In a digital tuning device for a PLL synthesizer, it is necessary to generate a digital code adapted to determine the frequency division number of a programmable frequency divider inserted in the PLL. Heretofore, for this purpose, a purely electronic method employing the combination of an up-down counter, a multivibrator, etc, has been proposed. Also a method using a memory board, such as a punch card, for storing binary information and a sensor for sensing the binary information out of the memory board have been studied. However, the former method is disadvantageous in that it is necessary to provide an upper and lower limit detecting circuit. As a result the manufacturing cost becomes higher than that of the latter method. In terms of the manufacturing cost, the digital tuning device according to the latter method is preferrable; however, it is still disadvantageous in the following aspects.
These will be described with reference to FIG. 1. A digital code storage perforated board 1 has perforations 3 through which light from a light source 2 passes to give a predetermined digital code to photo-transistors 6-1 through 6-N on a photo-transistor holder 4. The perforated board 1 may be planar as shown in FIG. 1 (a) or cylindrical; that is, the configuration of the perforated board 1 can be selected appropriately in accordance with the situation. When the photo-transistor holder is, for instance, in the position where the photo-transistors thereon face the line A or B of the perforated board 1, operation proceeds; that is, the relevant digital code can be sensed corresponding to its position without difficulty. However, when the photo-transistor holder is in the position where the photo-transistors thereon face the line C between the lines A and B, all of the bits of the digital code may be at an "L" level or the digital codes on both the lines A and B may be sensed.
In order to solve this problem, a method has been proposed in which a click board 5 is disposed below the perforated board 1 and a leaf spring 9 abutting against the teeth of the click board 5 is secured to the holder 4. By this technique, the photo-transistors 6 are moved only to predetermined positions. However, where this method is applied to a radio receiver, it is difficult to obtain the feeling of smooth operation which has been conventionally provided by the combination of a tuning knob and a flywheel. In addition, the method is disadvantageous in that the distance of digital pointer movement is shortened because the inertia of the flywheel is shortened. Hence the tuning operation becomes inconvenient.
Further, in a FM stereophonic receiver employing such a digital code generating method, a requirement exists to provide a muting circuit operating to open a gate only when the sensor faces a code on the perforated board and to correctly close the gate when the sensor is, for instance, placed between adjacent codes.